This invention relates to an apparatus for power factor control, for example, to a preconditioner of an electronic ballast and, more particularly, to a power factor controller of an electronic ballast preconditioner.
Power factor controllers control the ballast input current in response to the ballast input voltage in attempting to provide a ballast power factor of 1.0 (i.e. to make the ballast input look like a simple resistor). The ballast power factor is the ratio of the actual power of an alternating or pulsating current to the apparent power drawn by a ballast. When the input current relative to the input voltage contains phase displacement, harmonic distortion or both, the power factor will be less than 1.0. The amount of displacement between the voltage and current indicates the degree to which the load is reactive. Harmonic distortion, that is, the production of harmonic frequencies, arises from the nonlinear operation of the ballast in drawing power from a mains sinudoidal signal.
A conventional power factor controller often assumes that the input voltage to the ballast will be substantially sinusoidal. Non-linear loads (e.g. machines, motors) drawing power from the mains can temporarily distort the mains voltage resulting in a substantially nonsinusoidal input voltage. Control of the input current based on a non-sinsoidal input voltage can be difficult and can lead to power factor correction of far less than 1.0. It also can be difficult to maintain a total harmonic distortion (TBD) of less than 10% without adversely affecting the power factor.
Maintaining low THD while supplying a DC voltage to the ballast inverter with low ripple typically requires an undesirably large, expensive electrolytic capacitor. Such capacitors deteriorate with time (i.e. dry out) losing their capacitance and therefore their capability of maintaining low ripple. Unacceptable TED levels can result. The relatively large size of these electrolytic capacitors also hampers integrating the components of the ballast preconditioner onto an integrated circuit thereby limiting reduction in ballast manufacturing cost and size.
It is therefore desirable to provide an improved power factor controller which is unaffected by the temporary distortion of the mains voltage. The improved power factor controller should maintain a relatively high power factor (e.g. greater than 95%) without use of a relatively large electrolytic capacitor in order to maintain a low THD.